To develop energy harvesting technologies where electricity generated from small energy, such as sun light, vibrations, and body temperatures of human and animals, is stored to utilize for a power of sensors or wireless transmission, secondary batteries having high safety and reliability under various global environments are important.
As the secondary battery, a liquid-based battery using a liquid-based electrolyte is currently widely used.
The liquid-based battery however has a problem that a cathode active material is deteriorated to reduce a capacity of the battery, as a charging cycle is repeated, and a problem that short-circuit of the battery caused by a formation of dendrite may cause ignition of the organic electrolytic solution within the battery.
Therefore, the liquid-based battery lacks safety and reliability to be used for an energy harvesting device, a service life of which is expected to be 10 years or longer.
Accordingly, all solid batteries, all of constituting materials of which are solids, have been attracted attentions as the secondary battery having high safety and reliability. Advantages of the all solid battery are being free from leakage of a liquid and ignition, and having excellent cycle properties.
A solid electrolyte layer is used in the all solid battery. The solid electrolyte layer is typically obtained by compression-molding solid electrolyte particles, followed by firing. As the solid electrolyte particles, Li3xLa2/3-xTiO3 (0≤x ≤⅙) (LLTO) and Li7La3Zr2O12 (LLZO) are known to be materials having high lithium ion conductivity.
Although the solid electrolyte particles have excellent lithium ion conductivity within the particles, the resistance between the particles is significantly large compared to the resistance within the particle. Therefore, it is difficult to use the solid electrolyte particles as the solid electrolyte layer, as it is, even after the compression-molding of the solid electrolyte particles. In order to reduce the resistance between the particles, firing is performed after the compression molding. The firing is typically carried out at 1,000° C. or higher. To perform the firing has problems that costs for electricity and facilities are high.
In the case where an all solid battery is produced, moreover, it is effective to perform so-called integrated sintering, where sintering is performed with a cathode, a solid electrolyte, and an anode being in a combined state to reduce interface resistance between the electrolyte and the electrode. In the case where the LLTO and the LLZO are used as solid electrolytes, however, the integrated sintering needs to be performed at the sintering temperature of 1,250° C. or higher. Accordingly, the cathode and the anode, which do not cause melting or decomposition at a temperature of 1,250° C. or higher, need to be used, and thus there is a problem that a selection of materials for the cathode and anode is small.
Accordingly, various researches have been conducted to improve lithium ion conductivity of a solid electrolyte without firing.
For example, proposed is a lithium ion conductor, in which a hollow inorganic porous body is filled with an ionic liquid containing a lithium salt (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2013-30336). Moreover, a solid ion conductor containing an ordinary temperature molten salt and insulating inorganic particles is proposed (see, for example, JP-A No. 2011-119158).
In the proposed techniques, however, there is a possibility that the ionic liquid (the ordinary temperature molten salt) is leaked from the solid electrolyte under a high temperature environment, because the ionic liquid (the ordinary temperature molten salt) is used, and there is a problem in safety.
Moreover, proposed is electrolyte particles obtained by mechanically mixing electrolytic raw material crystal particles having lithium ion conductivity, and a lithium salt to cause distortion of the crystalline structure of the raw material crystal particles (see, for example, JP-A No. 2009-215130).
However, even the proposed technique has not attained sufficiently high lithium ion conductivity.
Accordingly, there is currently a need for a solid electrolyte, which excels in safety and reliability, can be produced at a low temperature, and has high lithium ion conductivity, as well as an all solid battery, which excels in safety and reliability, can be produced at a low temperature, and can achieve high output.